The invention relates to a 3D measuring system and to a method for heating such a 3D measuring system. The 3D measuring system comprises a projector, a camera and a chassis that connects the projector and the camera. Such 3D measuring systems are used to determine the 3D coordinates of objects.
EP 2 105 701 A1 discloses a 3D measuring system of this type. DE 10 2011 011 360 A1 discloses a further 3D measuring system of this type.
The projector of the 3D measuring system typically comprises a light source, in particular an LED, an optics and a pattern generator for generating a pattern that is to be projected onto the object, in particular a stripe pattern or a stochastic pattern. The camera typically comprises an optics and an image sensor having pixels that are arranged regularly in the manner of a surface, in particular a CCD sensor or a CMOS sensor. The camera is aligned with the region of the object onto which the pattern generator projects the pattern. It is possible for the 3D measuring system to have a plurality of projectors and/or cameras. The 3D measuring system can furthermore have further components, in particular a reference camera for recording one or more reference marks and/or sensors for a tracking apparatus.
Upon start-up of a 3D measuring system it warms up. As soon as the chassis has warmed up, the material of which the 3D measuring system is composed expands. It is also possible that parts of the chassis warp. As a result, the distance between the projector and the camera and the angle between the projector and the camera may change. If the 3D measuring system is conventionally heated in this manner, it is necessary to wait until a thermally stable state has been achieved. Once the system is thermally stable, the distance between the projector and the camera and the angle between the projector and the camera no longer change. Only at this point is it possible for the 3D measuring system to be calibrated in order to supply accurate 3D measurement data. If the decision is made to wait until all components of the 3D measuring system are thoroughly warmed up, this can take several hours, depending on the heat capacity of the system and of its components.
A significant proportion of the temperature increase in the 3D measuring system stems from the light source that is present in the projector and generates not only light, but also heat that must be transferred away. This can be done using a ventilation system. However, some of the heat also enters the chassis and the remaining mechanical parts of the 3D measuring system. Moreover, a large portion of the generated light is absorbed in the projector or lens of the projector, which contributes to the further heating of the 3D measuring system. In addition, further components of the 3D measuring system also generate heat, in particular the camera and circuit boards. This heat cannot be transferred away completely by the ventilation system.
In order to address the problem of thermal expansion, it is possible to use materials exhibiting very low thermal expansion, such as CRP, for constructing the 3D measuring system. In that case, the temperature of the chassis has less of an effect on the measurement result, since the chassis expands less and warps less. However, this solution goes hand-in-hand with the disadvantage of high costs, since components made of CRP and similar materials very often need to be produced manually. Due to the manual production, the process reliability also suffers. Furthermore, mechanical or thermal properties can deviate within a batch. Ultimately, it is not possible to replace all metal components by CRP or similar materials.
Another option is to separate components of the 3D measuring system in which high temperatures occur from the chassis using thermally insulating materials. An attempt could be made in this way in particular to thermally separate the projector including its light source from the chassis. However, the thermally insulating layers have only a relatively low mechanical strength. Consequently, they have a disadvantageous effect on the stability of the 3D measuring system.